Systematics of the charged-hadron P_T spectrum and the nuclear suppression factor in heavy-ion collisions from sqrt{s}=200 GeV to sqrt{s} =2.76 TeV

TL;DR

This paper performs a combined analysis of low- and high-P_T charged-hadron spectra in Pb-Pb collisions at the LHC, using models constrained by RHIC data, to understand the nuclear suppression factor and hadron production mechanisms.

Contribution

It extends hydrodynamic and pQCD-based models from RHIC to LHC energies, analyzing their compatibility with new ALICE data and exploring the discriminative power of high-P_T measurements.

Findings

High-P_T spectra are sensitive to model differences but limited by baseline uncertainties.

Hydrodynamic and pQCD models can describe the data with minimal additional mechanisms.

The transition region P_T=4-5 GeV is well explained by combined models.

Abstract

In this paper, our goal is to make a simultaneous analysis of the high- and low-P_T parts of the charged-hadron P_T spectrum measured by the ALICE collaboration in central Pb-Pb collisions at sqrt{s}=2.76 TeV at the Large Hadron Collider (LHC), based on models which have been successfully applied and constrained in Au-Au collisions at Relativistic Heavy Ion Collider (RHIC). For the hydrodynamical modeling with which we obtain the low-P_T spectrum, we have computed the initial conditions based on perturbative QCD (pQCD) minijet production and saturation. The sensitivity of the obtained charged-hadron P_T spectrum on the hydrodynamic model parameters is studied. For the high-P_T part, we apply a number of parton-medium interaction models, which are tuned to describe the nuclear suppression factor R_AA measured at the RHIC in central Au-Au collisions at sqrt{s}=200 GeV. We find that the higher kinematic reach of the LHC, manifest in the hardening of the pQCD parton spectral slope, is in principle very efficient in discriminating the various models. However, due to the uncertainties in the p-p baseline, none of the tested models can be firmly ruled out with the present ALICE data. Comparison with the LHC data in this approach also shows that the matching of the hydrodynamic and pQCD+jet quenching components leaves fairly little room for other hadron production mechanisms in the cross-over region P_T=4-5 GeV.